This invention relates to membrane probing of circuits.
Circuits to be probed may include those which are formed as part of "dice" arranged in rows and columns on a silicon wafer during the process of making integrated circuits (IC's). Each die includes as many as a hundred or more aluminum input/output (I/O) pads for connecting the circuit on the die to the outside world. The pads are typically tiny and closely spaced, e.g., on the order of 100 microns center-to-center (pitch).
The performance and/or integrity of the dice circuits may be tested before the dice are cut out of the wafer and packaged. Testing is typically done by conductive probes that make physical and electrical contact with the die pads. Generally, the probes are either hard pins (e.g., tungsten), or hard bumps formed on a membrane surface. The close spacing of the die pads requires that the probes be carefully aligned, often by hand or by using a special visual alignment system. During testing, the probes are forced into contact with the pads.
Often, the sharp probe tips are wiped across the pads to abrade or scratch a thin layer of insulating aluminum oxide (which sometimes forms on the pads), thus exposing non-oxidized portions of the pads. Because the probe tips become dirty from this and other processes, they are periodically cleaned with an abrasive such as sandpaper.
During testing, the probes relay the appropriate power, data, and other signals between the die and a tester. The frequency at which these test signals are communicated (and thus also the testing rate) is determined at least in part by the inductance of the probes and the impedance of the electrical interconnections between the die and the tester.